Another study found that a year of resistance training, once or twice a week, improved older women's attention spans and conflict resolution skills.

A third found that adults aged 55 and older who engaged in moderate or high physical activity were less likely to become cognitively impaired than their couch-potato equivalents. And women aged 65 or older who took part in an exercise program for 18 months appeared to have denser bones and a reduced risk of falls than women the same age who followed a less intense "wellness" program, a fourth study showed.

The findings of the studies, which were conducted in Canada, Germany and the United States, could be just what the doctor ordered to get more Americans to exercise, the authors of the first study said.